What is Digestion?
Before we describe about the digestion of carbohydrates let us have some idea about the meaning of digestion. Most of our foods are complicated chemical structures. It is very easy to understand that in order to be absorbed and utilized by the body, the food must be broken down into comparatively simple molecules. Carbohydrates must be hydrolyzed into monosaccharide, fats into glycerol and fatty acids, and proteins into amino acids. The process by which the complex food materials are changed to simple molecules is called digestion.
Digestion of carbohydrates
The sucrose (cane sugar), lactose (milk sugar) and starches are the three major sources of carbohydrates in the normal human diet. Amylose, glycogen, pectins and dextrins, and minor quantities of other carbohydrate derivatives are the other carbohydrates ingested to a small extent. The human diet also contains a large amount of cellulose, but enzymes capable of hydrolyzing cellulose are not secreted by the human digestive system. Hence, cellulose is not considered a food for human beings. But for many herbivorous animals cellulose is the principal source of carbohydrates.
Digestion of carbohydrates in buccal cavity and stomach
When food is chewed, it is mixed with saliva, which contains the enzyme ptyalin, which is a-amylase secreted mainly by parotid glands. The enzyme ptyalin hydrolyses starch into dextrins, maltotriose and lastly into maltose. The salivary amylase is most active at pH 6.8 and is readily inactivated at pH 4.0 or below. Thus the digestive action soon ceases in the acidic environment of the stomach. As food remains in the mouth only for a short time, i.e., not more than 3-5 per cent of all the starches are hydrolyzed in the mouth by the time the food is swallowed by human beings. So, digestion of carbohydrates continues in the stomach for as long as an hour until the food is mixed with the secretions of stomach. After that, the activity of salivary amylase is blocked by the acid of gastric secretions. Before the food becomes completely mixed with gastric secretions, 30-40 per cent of the starches are hydrolyzed mainly to maltose.
Since hydrochloric acid present in gastric juice destroys all ptyalin within a short time, the gastric juice contains no carbohydrate-digesting enzyme.
Digestion of carbohydrates in intestine
The chyme, as it leaves the stomach, passes into the small intestine. In duodenum, it is mixed with bile, pancreatic juices and intestinal juices. Though bile has no enzyme and as such it has no chemical action on carbohydrates. Pancreatic juice contains a starch-digesting enzyme pancreatic amylase. It acts optimally at a neutral to slightly alkaline medium and is several times powerful than saliva amylase. Thus within 15-30 minutes after the chyme mixes with pancreatic juice in the duodenum virtually all the starches are digested. They are almost totally converted into maltose and other very small glucose polymers before they have passed beyond the duodenum.
In the ordinary diet, which contains far more starches than sucrose or lactose, glucose represents more than 80 per cent of the final products of digestion of carbohydrates. The human being is the only mammal that digests significant amounts of lactose in milk even as an adult is the part of digestion of carbohydrates.
Vertebrates do not produce any enzyme to digest cellulose. They depend on symbiotic digestion of cellulose by micro-organisms harbored in their digestive tracts. These micro-organisms are found in the humen and reticulum of the compound stomach of ruminants and in the caecum and vermiform appendix of non-ruminants. These micro- organisms secrete cellulaseenzyme that ferments cellulose into short chain fatty acids like acetic acid and propionic acid. These fatty acids are then absorbed and utilized by the host animal.